MUSEUM OF COMPARATIVE ZOOLOGY. 123 



subject to slight individual variations, but the number of such cells is 

 nevertheless in quite close correlation with the stage of development. An 

 examination of Goette's Figures G-9 ('87, Taf. I.) reveals such a siuii- 

 larity in the number and size of tlie cells composing the ectoderm in each 

 of the four supposed stages, that I am driven to the conclusion that 

 they represent sections from specimens of a single stage of development, 

 which may have been produced by cutting in planes having difterent 

 relations to the chief axis of the embryo. 



When we consider that in the majority of embryos there are no signs 

 of ingression, and that in the cases where it does occur the immigrating 

 cells in some instances degenerate early, and in others persist undivided 

 throughout the process of gastrulation, and that they at no time show evi- 

 dences of even sharing in the formation of an entoderm, — and when we 

 further reflect that all the conditions shown in Goette's Figures 6-9 can 

 easily be reproduced fi'om sections of invaginating gastrulse of a single 

 stage of development, — it seems improbable that the entoderm of Au- 

 relia develops even occasionally by ingression. At present, therefore, 

 there seems to me to be no evidence that in this genus gastrulation 

 occurs by both methods, invagination and ingression. 



The Scyphomedusse present several interesting variations in gastru- 

 lation. The anomalous development occurring in Lucernaria is as far 

 removed from the usual process as that group itself is from the other 

 Scyphomedusse. According to McMurrich ('91, p. 314), the solid plan- 

 nla in Cyanea arctica is formed by the immigration of certain of the 

 blastula cells. This plauiila is subsequently hollowed out, and gives 

 rise to a structure like an invaginate gastrula, but it is formed without 

 any invagination. In Cyanea capillata (Hamann, '90, pp. 16, 17) there 

 seems to be a solid ingrowth of cells from one pole of the embryo, and a 

 simultaneous development of the coelenteron. The entoderm of Chry- 

 saora (Clans, '83, p. 5, Taf. I. Fig. 21 k) is developed in away which is 

 somewhat similar to that described by Hamann for Cyanea capillata. 

 According to Clans ('83, p. 2, and '90, p. 4), the gastrulation of Aurelia 

 aurita approximates the method by invagination a little more closely 

 than that of Chrysaora, since its cells are arranged in a single layer 

 about the fissure-like coelenteron. Aurelia flavidula exhibits a still more 

 nearly typical invagination, since the coelenteron is from the beginning 

 an open sac-like cavity. Cotylorhiza tuberculata (Cassiopea Borbonica) 

 lias an invaginate gastrula which closely resembles that of Aurelia 

 flavidula (Claus, '90, Taf. I. Figs. 2 and 3 ; Kowalevsky, '73, Taf. II. 

 Fig. 1). Finally, in Pelagia noctiluca and Nausithoe marginata, as 



